Ultrahydrophobic silica films by sol–gel process

Wettability of solid surfaces is a crucial concern in our daily life as well as in engineering and science. The present research work describes the room temperature (27 °C) synthesis of adherent and water repellent silica films on glass substrates using vinyltrimethoxysilane (VTMS) as a hydrophobic reagent by a single step sol–gel process. The silica sol was prepared by keeping the molar ratio of tetraethoxysilane (TEOS), methanol (MeOH), water (H₂O) constant at 1:14.69:5, respectively, with 0.01 M NH₄F throughout the experiments and the VTMS/TEOS molar ratio (M) was varied from 0 to 0.97. The effects of M on the surface structure and hydrophobicity have been researched. The static water contact angle as high as 144° and water sliding angle as low as 14° was obtained for silica film prepared from M = 0.97. The hydrophobic silica films retained their hydrophobicity up to a temperature of 255 °C and above this temperature the films became superhydrophilic. The prepared silica films were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Fourier Transform Infrared (FT-IR) spectroscopy, humidity test and static and dynamic water contact angle measurements.     

Preparation of scratch resistant superhydrophobic hybrid coatings by sol–gel process

Organic–inorganic hybrid coatings on glass substrates with superhydrophobic properties and with improved scratch resistance were obtained by means of applying a multilayer approach including multiple sol–gel processes. The coatings exhibited a water contact angle (WCA) higher than 150°. Ultraviolet (UV)-curable vinyl ester resins and vinyltriethoxysilane (VTEOS) as coupling agent were employed to increase the adhesion between substrate and the inorganic layers. The surfaces were characterized by means of dynamic contact angle and roughness measurements. Indeed, the occurrence of superhydrophobic behavior was observed. The scratch resistance of the hybrid coatings was tested to evaluate the adhesion of the coatings to the glass substrate. The proposed preparation method for scratch resistant, mechanically stable, superhydrophobic coatings is simple and can be applied on large areas of different kinds of substrates.     

Preparation of hollow mesoporous silica spheres by a sol–gel/emulsion approach

Hollow mesoporous silica spheres were synthesized by a sol–gel/emulsion (oil-in-water/ethanol) approach, in which cetyltrimethylammonium bromide (CTAB) surfactant was employed to stabilize and direct the hydrolysis of oil droplets of tetraethoxysilane (TEOS). The diameters of the hollow spheres can be tuned in the range from 210 to 720 nm by varying the ratio of ethanol-to-water and their shell thickness can be mediated by changing the concentration of CTAB used in the system. BET surface areas of the hollow silica spheres are determined to be in the range of 924–1766 m² g^?1 and their pore sizes are around 3.10 nm as determined by BJH method.     

Preparation and dielectric properties of dense and amorphous alumina film by sol–gel technology

Dense and crack-free aluminum oxide films were fabricated by sol–gel spin-coating technology. Aluminum nitrate (Al(NO₃)₃.9H₂O) was used as the precursor material. X-ray diffraction shows that the fabricated films are amorphous. X-ray photoelectron spectroscopy confirms that the thin films are alumina (Al₂O₃). Field-emission scanning electron microscopy images of the films reveal that the films are compact with a dense cross section. Dielectric measurements were carried out on samples with a metal–insulator–metal structure. The electrical characteristics of the films were affected by the thermal sintering temperature of the films. The leakage current density of the films decreases with the increase in the sintering temperature and increases with the increase in the measuring temperature. The leakage current shows a linear dependence on the voltage in the low-electric field-regime. The current density ascends to higher values due to the effect of space charges in the high-electric-field regime. The ionization energy of the top-electrode metals (Au, Pt or Ti–Au) has a strong effect on the leakage current.     

Highly conductive coatings of carbon black/silica composites obtained by a sol–gel process

Conductive submicronic coatings of carbon black (CB)/silica composites have been prepared by a sol–gel process and deposited by spray-coating on glazed porcelain tiles. Stable CB dispersions with surfactant were rheologically characterized to determine the optimum CB-surfactant ratio. The composites were analyzed by Differential Thermal and Thermogravimetric Analysis and Hg-Porosimetry. Thin coatings were thermally treated in the temperature range of 300–500 °C in air atmosphere. The microstructure of the coatings was determined by scanning electron microscopy and the structure evaluated by confocal Raman spectroscopy. The electrical characterization of the samples was carried out using dc intensity–voltage curves. The coatings exhibit good adhesion, high density and homogeneous distribution of the conductive filler (CB) in the insulate matrix (silica) that protects against the thermal degradation of the CB nanoparticles during the sintering process. As consequence, the composite coatings show the lowest resistivity values for CB-based films reported in the literature, with values of ～7 × 10^?5 Ωm.     

Preparation of macroporous cordierite monoliths via the sol–gel process accompanied by phase separation

Monolithic cordierite with a cocontinuous macroporous structure has been successfully prepared by the sol–gel process accompanied by phase separation in the presence of poly(acrylamide) (PAAm). Propylene oxide (PO) acts as an acid scavenger to mediate the gelation of MgO–Al₂O₃–SiO₂ ternary system, while PAAm works as a phase-separation inducer as well as a network former. The dried gel and that heat-treated at 800 °C are amorphous, and the sapphirine begins to precipitate at 900 °C, then transforming to orthorhombic β-cordierite at 1100 °C. After heat-treated at and above 1200 °C, the resultant β-cordierite further transforms to stable hexagonal α-cordierite. Heat-treatment changes the macroporous structure of cordierite monoliths such as macropore size and its distribution. The macroporous cordierite monolith after heat-treated at 1200 °C is found to possess a total porosity of 54%, interconnected macropores and dense solid skeletons.     

Effect of silica sol on the dispersion–gelation process of concentrated silica suspensions for fibre-reinforced ceramic composites

Sol gel process of silica powders dispersed in silica sol has been used to obtain a suitable suspension for the sol infiltration technique of fibre-reinforced ceramics. Efforts have been focused on analysing the effects of polyelectrolyte content, pH, solid load and concentration of gelling agent on the flow behaviour of silica suspensions. The most adequate suspension to manufacture the matrix of composites is a weak flocculated suspension with negligible thixotropic behaviour at pH ≥ 9.5, which is composed of silica microparticles dispersed in silica sol with 41 vol.% of solid load and 1.8 wt.% of Duramax D3005. This suspension easily undergoes a transition to a gel by a slight alteration of stability conditions adding 0.08 M of NH₄Cl, which reduces both the electrostatic repulsion and the pH to 8. Moreover, silica sol promotes the densification of ceramics up to 70% after sintering at 900 °C, allowing porous matrix processing of the composites.     

Preparation and characterization of silica sol/fluoroacrylate core–shell nanocomposite emulsion

The silica sol/fluoroacrylate core?Cshell nanocomposite emulsion was successfully synthesized via traditional emulsion polymerization through grafting of KH-570 onto silica particles. Comparing the performance of the polyacrylate copolymer, the fluorinated polyacrylate copolymer and the silica sol/fluoroacrylate core?Cshell nanocomposite emulsion, we can come to a conclusion that the silica sol/fluoroacrylate core?Cshell nanocomposite emulsion presents significantly excellent performance in all aspects. The products were characterized by Fourier transform infrared (FTIR), photon correlation spectroscopy (PCS), transmission electron microscopy (TEM), thermogravimetry (TGA), Contact angle and UV?Cvis analyses techniques. The chemical structure of polyacrylate copolymer, fluorinated polyacrylate copolymer and silica sol/fluoroacrylate nanocomposite were detected by FTIR. The size and stability of emulsion latex particles were determined by PCS technique. TEM analysis confirmed that the resultant latex particle has the core?Cshell structure, obviously. The water absorption and contact angle data also showed that the silica sol/fluoroacrylate nanocomposite film has good hydrophobic performance. TGA analysis indicated the weight loss of the silica sol/fluoroacrylate nanocomposite film begins at around 350?°C which testifies its good thermal stability. The UV?Cvis spectroscopy analysis showed that the silica sol/fluoroacrylate nanocomposite film possess UV?Cvis shielding effect when the added volume amount of KH570 modified silica sol is up to 5?mL. Therefore, the excellent properties of hydrophobicity, thermodynamics and resistance to ultraviolet provide the silica sol/fluoroacrylate nanocomposite film with potential applications in variety fields. In addition, the formation mechanism of core?Cshell structure silica sol/fluoroacrylate nanocomposite latex particles was speculated.     

Preparation of dendritic–linear polyether-modified silica sol and its application in coatings

Dendritic–linear polyether-modified silica sol (DLPS) was synthesized using diethylenetriamine, methyl acrylate, epoxy-terminated polyether, triethoxysilane, and silica sol in five steps. The prepared sol was stable and transparent. DLPS was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and particle size distribution analysis. Subsequently, DLPS was examined as a stain-resistant additive in (paint) coatings. The surface of the sample coatings with and without DLPS additive was examined by scanning electron microscopy, energy-dispersive spectrometry analysis, and contact angle analysis. The results showed that the stain-resistant additive appeared on the surface of the coating, which made the surface more compact and hydrophilic. The performance of the stain-resistant additive was examined in commercial outdoor emulsion coatings (WB200). The result showed that the presence of the stain-resistant additive could enhance the stain resistance performance of paint. Furthermore, the stain resistance performance improved with increasing amounts of additive. Specifically, the stain resistance performance of paint improved by 40.63% in the presence of 6 wt% DLPS. Therefore, the current findings demonstrate the importance of DLPS in advancing progress and application in the paint coating industry.     

Fabrication of durable superhydrophobic and superoleophilic cotton fabric with fluorinated silica sol via sol–gel process

Superhydrophobic and superoleophilic cotton fabric was successfully prepared with fluorinated silica sol via a facile sol–gel method. A fluorinated polymeric sol–gel precursor (PHFBMA-MTS) was synthesized via free-radical polymerization by using hexafluorobutyl methacrylate (HFBMA) in the presence of (3-mercaptopropyl)trimethoxysilane (MTS) as the chain transfer agent, which led to the formation of fluoropolymer with alkoxysilane end groups. Then the fluorinated silica sol was prepared by introducing PHFBMA-MTS as the co-precursor of tetraethylorthosilicate (TEOS) in the sol–gel process with ammonium hydroxide as the catalyst, which was then used to fabricate superhydrophobic and superoleophilic fabric coatings through a simple dip-coating method. The coated fabrics showed superhydrophobic property with a high water contact angle of 154.1° and superoleophilic property with an oil contact angle of 0°. Moreover, the coated fabrics still kept superhydrophobicity even after ultrasonic treatment, as well as for organic solutions, acidic solutions. Thus, the coated fabrics were successfully applied to separate oil–water mixture with separation efficiency up to 99.8%. More importantly, the separation efficiency had no significant change after 20 cycles of oil–water separation. These present a simple, low-cost, and durable approach to achieve industrialized application of coated fabrics in oil–water separation. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47005.     

Preparation and characterization of phosphine oxide containing organosilica hybrid coatings by photopolymerization and sol–gel process

A series of UV-cured organic–inorganic hybrid coating materials containing up to 20 wt.% silica were prepared by sol–gel method from tetraethoxy silane (TEOS) which is used as the primary inorganic precursor, and diallylphenylphosphine oxide monomer (DAPPO), aliphatic urethane diacrylate resin (Ebecryl 210) are employed as the source of the organic components. In addition, methacryloxypropyltrimethoxy silane (MAPTMS) was used as both a secondary inorganic source and a silane-coupling agent to improve the compatibility of the organic and inorganic phases. The DAPPO content in all the coating formulations were from 0 to 20 wt.%. The physical and mechanical properties such as gel content, hardness, adhesion, gloss, contact angle as well as tensile strength were measured. These measurements revealed that all the properties of the hybrid coatings improved effectively, in case of adding the sol–gel precursor and DAPPO monomer content in the hybrid systems. The photo-calorimetric-DSC studies showed that the double bond conversion of the hybrid coatings was faster than the coating materials without silica. The thermal stabilities of the UV-cured hybrid materials were investigated by thermogravimetric analysis. The results showed that the addition of sol–gel precursor and DAPPO into the organic network also improves the thermal-oxidative stability of the hybrid coating materials. The surface morphology was also characterized by scanning electron microscopy (SEM). SEM studies indicated that inorganic particles were dispersed homogenously throughout the organic matrix.     

Hydrophilization by coating of silylated polyoxazoline using sol–gel process

Hybrid films prepared from TEOS and polyoxazolines (Si–POx–Si) crosslinking agents were coated on different substrates in order to modify their surface properties. The film cohesion and adhesion on substrates were expected through the hydrogen bonding of the polyoxazoline crosslinked network. Low molecular-weight α,ω-unsaturated polyoxazolines (DA-PMOx)s were synthesized by a one step cationic ring-opening polymerization (CROP) of 2-methyl-2-oxazoline (MOx) with a good control over the molecular weight. Based on double thiol-ene coupling (d-TEC) a post-functionalization of DA-PMOx end chains gave in good yield polyoxazoline cross linker (Si–POx–Si). Glass and various polymer substrates (PP, PEI, POM, etc.) were spin coated by the organic–inorganic hybrid films through sol–gel process. AFM, SEM, visible reflectance spectroscopy and contact angle experiments allowed the full characterization of targeted surfaces and demonstrated the efficiency of the polyoxazoline coating.     

Preparation of hierarchically porous spinel CoMn2O4 monoliths via sol–gel process accompanied by phase separation

Cobalt manganite-based hierarchically porous monoliths (HPMs) with three-dimensionally (3D) interconnected macropores and open nanopores have been prepared via the sol–gel process accompanied by phase separation. The controlled hydrolysis and polycondensation of the brominated metal alkoxides, which are generated from an incomplete reaction between epichlorohydrin and MBr₂ (M = Co and Mn) in N,N-dimethylformamide (DMF), form a monolithic gel based on the two divalent metal cations. The dual-polymer strategy using polyvinylpyrrolidone (PVP) and poly(ethylene oxide) (PEO) effectively induces the spinodal decomposition, where PVP and PEO are preferentially distributed to the gel phase and fluid phase, respectively, resulting in a porous gel characterized by the co-continuous structure. The effects of DMF and PVP on the porous morphology derived from the phase separation have been systematically studied. Calcination of the as-dried gels allows for the crystallization into the spinel phase yielding hierarchically porous CoMn₂O₄ monoliths, which have been examined in detail by the structural and compositional analyses.     

Preparation and antibacterial activity of Ag/SiO2 thin film on glazed ceramic tiles by sol–gel method

In the present study, silver-doped silica thin films on glazed surface of ceramic tiles were well prepared by sol–gel method to achieve antibacterial activity. Thermal treatment was done in the air at 1100 °C for two hours. The Ag/SiO₂ thin films were investigated through Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and wavelength dispersive spectrometry (WDS). Atomic absorption spectroscopy (AAS) was used for the quantitative determination of the silver ion concentration being released from Ag/SiO₂ films over a 24 day period. The antibacterial effects of Ag/SiO₂ thin films against Escherichia coli and Staphylococcus aureus were also examined. From the analysis results, it was found that high temperature treated coating consists of two phases of SiO₂ and Ag based on the trapping of the Ag phase in the silica matrix. The presence of Ag elements on the surface of the coated tiles, were also observed. Thermal treatment at high temperatures caused sharp XRD peaks and high crystallinity in this system. Ag⁺ ions were released constantly and the mean release rate (±SD) was 0.104 ±0.01 μg/ml during 24 days. Coating films exhibited an excellent antibacterial performance against both bacterium.     

Structure and optical properties of transparent Er3+-doped CaF2–silica glass ceramic prepared by controllable sol–gel method

Transparent Er³⁺-doped CaF₂–silica glass ceramics were prepared by the direct physical introduction of Er³⁺ doped CaF₂ nanocrystals into acid-catalyzed sol–gel silica glass. The physical methods of ball milling, ultrasonic baths, and stirring were investigated to disperse Er³⁺ doped CaF₂ nanocrystals in the silica sols. The CaF₂–silica sol mixture went through gelation and heat-treatment to form Er³⁺-doped CaF₂–silica glass ceramics. The morphology of Er³⁺ doped CaF₂ in silica glass did not change after heat-treatment at 600 °C for 10 h. The experimental results showed that Er³⁺ doped CaF₂ in the glass ceramic prepared with the assistance of ball milling possesses the best dispersity and homogeneity. The highest in-line transmittance of the glass ceramic reached up to 85% in visible region. Glass ceramic exhibits efficient up-conversion emissions corresponding to the Er³⁺:⁴F_9/2→⁴I_15/2 transition and long lifetime of ⁴F_9/2 level (1.73 ms) under 980 nm excitation.     

Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method

Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H₂SO₄ at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m²/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.     

Preparation of nanocrystalline SrTiO3 powder by sol–gel combustion method

In this research a sol–gel combustion route has been presented to synthesize strontium titanate (SrTiO₃:ST) nanocrystalline, using citric acid as fuel. The synthesis procedure was optimized by systematically varying the molar ratios of total metal nitrate to citric acid (MN:CA) from 1:1 to 1:3. The effect was investigated through XRD, SEM and TEM analysis. Analysis of XRD spectrum shows the complete of SrTiO₃ nanocrystalline, however, a minor phase of SrCO₃ impurity was found. Hence, an acid treatment process, with 1 mol/l HNO₃ solution and deionized water, was applied to remove the impurity. The results show that the appropriate condition to prepare the single phase nanocrystalline SrTiO₃ powders is MN:CA molar ratio of 1:3, coupled with an acid treatment process and at the lower calcination temperature of 500 °C. The particle size of powders was in nanometer ranges. The average crystallite size calculated from FWHM was about 23 nm. Morphology of powders was identified by SEM analysis. However, TEM estimated the average particle size about 7.5 nm after applying an acid treatment technique at 600 °C.     

Preparation of mullite monoliths with well-defined macropores and mesostructured skeletons via the sol–gel process accompanied by phase separation

Mullite monoliths with well-defined macropores and mesostructured skeletons have been prepared via the sol–gel process accompanied by phase separation in the presence of poly(ethylene oxide) (PEO). Gelation of Al₂O₃–SiO₂ binary system with chloride salts as an additional precursor has been mediated by propylene oxide (PO) as an acid scavenger, while PEO worked as a phase-separation inducer. The dried gel and that heat-treated at 800 °C are amorphous, and γ-Al₂O₃ or Si–Al spinel phase nanocrystals are crystallized at 900–1000 °C. After heat-treated at and above 1100 °C for 5 h, the complete crystalline mullite is generated, and the macroporous monoliths in large dimensions of more than 15 mm × 15 mm × 10 mm are obtained. Heat-treatment at 200–1400 °C does not basically spoil the macroporous structure of monoliths, while decreases the macropore size and significantly alters the phase compositions and micro-mesoporous structure.     

Preparation of hybrid silica sol–gel coatings on mild steel surfaces and evaluation of their corrosion resistance

Hybrid silica sol–gel coatings were prepared on mild steel substrate by dip coating technique. The coatings were subsequently heat treated at 200 °C in order to improve their corrosion properties. The coating sols were synthesized using Glycidoxypropyltrimethoxysilane (glymo) and Aminopropylethoxysilane (ameo) as precursor materials. Potentiodynamic polarization curves were derived and Electrochemical Impedance Spectroscopy (EIS) measurements were made in NaCl solution. The surface and cross-section morphology of coated specimens were characterized by scanning electron microscopy (SEM). Fourier transformed infrared (FTIR) analysis was used to identify the presence of various functional groups in the coating solutions. A comparison of the corrosion resistance of the coated and uncoated mild steel was presented. The results indicated that the corrosion resistance of the coated mild steel was improved considerably.